Yes, given the litigious nature of this country, there's no doubt that "in the real world," folks would be lining up to sue the Avengers for the Battle of Manhattan. But I strongly doubt Marvel Studios or audiences want to get bogged down in that level of boring courtroom drama. And from a screenwriting standpoint, I'd say that Stark Industries has enough legal resources to shoot those lawsuits down; plus Nick Fury has a vested interest in the Avengers, and would certainly bring all his considerable political power to bear in Tony's favor.

IM3 has enough on its plate already; let's not bring the lawyers in, too.

Well that doesn't have to stop them from briefly mentioning the lawsuits on TV or something...Tony could be watching it on TV and turn it off or something like in IM2